Bipolar junction transistors (BJTs) are commonly used in digital and analog integrated circuit (IC) devices for high frequency applications. A BJT includes two p-n junctions sharing a cathode or anode region, which is called the base. The base separates two regions having a same conductivity type, called the emitter and the collector, which is opposite of the conductivity type of the base. Depending on the conductivity types, a BJT can be of the NPN variety or the PNP variety.
A heterojunction bipolar transistor (HBT) is a type of BJT that uses different semiconductor materials for the emitter/collector and the base. By using different materials, the HBT reduces injection of holes from the base into the emitter region over the BJT. Consequently, the HBT supports higher frequencies than a BJT (e.g., several hundred GHz).